A conventional packaged semiconductor laser device is shown in FIG. 1. This packaged semiconductor laser device comprises a laser diode element 1, a mount 2 on which the laser diode element 1 is mounted, a heat sink 3 comprising, for example, Ag or Au, for absorbing heat generated by the laser diode element 1, a stem 4 on which the heat sink 3 is mounted, and a cap 5 with a hole formed in its top wall placed over the stem 4 for hermetically sealing the device. Laser light goes out through the hole. In other conventional devices, for example, MITSUBISHI InGaAsP laser diode ML8611 or ML8701, a glass plate transmissive to laser light is mounted in the hole. When such a conventional device is coupled to, for example, an optical fiber of an optical communications system, part of the laser light may be reflected back to the laser device from the joint between the semiconductor laser device and the optical fiber, pass through the glass plate and return to the laser diode. Such reflected light disturbs stable oscillations of the laser diode, which could result in undesirable variations in a current-voltage characteristic, an increase of the number of longitudinal modes, an increase in noise etc. In order to prevent reflected laser light from returning to the laser diode, the device shown in FIG. 1 includes an optical isolator comprising a pair of polarizers 61 and 62 formed of, for example, calcite, and a Faraday rotator 10 disposed between the polarizers 61 and 62. The polarizers 61 and 62 with the Faraday rotator 10 disposed therebetween are mounted in the hole. A permanent magnet 11 for the Faraday rotator 10 is also mounted in the hole. The directions of polarization of the pair of polarizers 61 and 62 are angularly displaced by 45.degree. from each other.
In operation, laser light emitted by the laser diode 1 passes through the polarizer 61 and then through the Faraday rotator 10. As the light passes through the Faraday rotator 10, its plane of polarization is rotated by 45.degree.. Then, the rotated laser light goes out through the other polarizer 62. A part of the laser light which has passed through the polarizer 62 may be reflected back to the semiconductor device from, for example, a joint between the semiconductor laser device and an optical fiber external to the laser device. This reflected light again passes through the polarizer 62, has its plane of polariation further rotated by 45.degree. by the Faraday rotator 10 and arrives at the other polarizer 61. Thus, the plane of polarization of this reflected light after it has passed the polarizer 61 has been rotated by 90.degree. relative to that of the initially emitted light, and, therefore, the reflected light cannot pass through the polarizer 61 and is effectively blocked. Thus, the semiconductor laser device of the FIG. 1 is free of adverse effects of returning laser light and can operate stably.
However, since the conventional semiconductor laser device described above uses the permanent magnet 11, a magnetic field is generated by the magnet 11, which could cause contamination of the device by foreign particles. Furthermore, this conventional device cannot be used in a system which cannot properly operate in the presence of a magnetic field. In addition, this conventional semiconductor laser device is expensive, because it employs a Faraday rotator.